During the normal operation of a television receiver, the energy of the electron beam is distributed across a relatively large area of the picture tube screen. When the receiver is turned off or switched to a stand-by operating mode, the scanning currents in the horizontal and vertical deflection windings may collapse before the energizing potential for various electrodes of the picture tube decay sufficiently to prevent the generation and acceleration of the electron beam, and the concentration of beam energy may damage the kinescope phosphor or produce undesired visual artifacts such as after-glow.
An example of a television receiver having run and stand-by operating modes and which includes provisions for kinescope spot burn protection during a transition from the run or normal viewing mode to a stand-by mode (in which some receiver circuits remain energized) is described by Peter E. Haferl in U.S. Pat. No. 4,488,181 entitled ELECTRON BEAM SUPPRESSION CIRCUIT FOR A TELEVISION RECEIVER which issued Dec. 11, 1984.
In an exemplary embodiment of the Haferl receiver, a deflection generator is coupled to the deflection winding for generating scanning current to produce a sweep of the electron beam across the phosphor screen of a kinescope. A remote control circuit develops an on/off command signal for switching the television receiver between normal run and stand-by operating modes. A first switch is responsive to the command signal and disables normal generation of the scanning current upon the occurrence of the off-state of the command signal in inhibit normal sweep of the electron beam. A second switch, also responsive to the command signal, applies to the grid electrode of the picture tube, a blocking potential upon the occurrence of the off-state of the command signal to suppress generation of the electron beam prior to the disablement of scanning current generation.
The Haferl technique of applying grid cut-off bias to the kinescope when switching the receiver from on to off is an excellent and effective method of preventing kinescope spot burn. It is a further feature of the Haferl receiver that provisions are also made for suppression of an after-glow effect under a so-called "hot start" receiver operating condition. Specifically, Haferl includes provisions for delayed build-up of screen grid voltage. This delay ensures that no beam spot becomes visible even should the television receiver be turned on while the cathode electrodes are still hot and while a substantial ultor voltage still remains. Such a situation may occur when the receiver is rapidly cycled between standby and normal running modes of operation (i.e., a "hot start" condition).